LASIK eye surgery is done by manipulating the corneal diopteric power and for this many variables are to be considered and fed into the machine and software calculates the parameters. All these parameters are not absolute and some are even beyond calculations, so a small error of 0.25D to 0.5D is inevitable and moreover this small error is not visually significant, it hardly makes a difference of 1 line on vision testing chart.

0.50 variation is a very subjective variation and so it isn't considered significant. If your power is checked on two consecutive days, there is a possibility that on one day you might accept 0.50 difference and on another day you won't.